1. Field of the Invention
The present invention relates to combination transmitting and receiving, capacitance type, electrostatic transducers capable of transmitting and receiving an ultrasonic object detection signal in general, and to such transducers for use with ultrasonic range-finding systems, in particular.
2. Description of the Prior Art
Ultrasonic ranging systems for focusing the lens of a photographic camera have been disclosed in the prior art. Copending patent application Ser. No. 3,371, filed Jan. 15, 1979, by J. MUGGLI, discloses a ranging system for focusing the adjustable focus lens of a camera in response to the transmission and reception of a single burst of multiple frequency, ultrasonic energy. This arrangement enables a camera operator to sequentially range, focus and actuate a camera's shutter mechanism in a relatively short period of time, as compared to human reflux time, in response to the manual depression of a shutter release button.
The ranging system disclosed in the above-cited application utilizes a combination transmitting and receiving, capacitance type, electrostatic transducer for both transmitting and receiving the multiple frequency burst of ultrasonic energy mentioned above. To be practical for use in camera focusing however, the transducer in such a camera ranging system must have a high mechanical damping factor in order to insure rapid decay of transducer diaphragm vibrations after termination of a transmit or transducer drive signal before an echo of said transmit signal reaches said transducer. A transducer diaphragm that continues to vibrate or "ring" for an excessive period of time after the termination of a transmit signal will erroneously appear to be a true echo of said transmit signal to said camera ranging system, which may result in camera lens misfocusing. As a result of this "ringing" phenomenon, the closest object detection distance of such a system is dependent upon the time required for the vibrations of a vibrating transducer diaphragm to decay after a transmit or transducer drive signal has been terminated.
A capacitance type electrostatic transducer capable of transmitting ultrasonic energy and sensing a reflection or echo of said transmitted energy is described in U.S. Pat. No. 4,081,626 to MUGGLI, et al. In such a transducer, a thin plastic film, metallized on one surface to form an electrode, is stretched over a relatively massive metallic counter-electrode, hereinafter termed the backplate, with the non-conductive surface of said film in contact with said backplate. The metallized surface of the film separated by the insulating film from the backplate defines a capacitor such that when a dc bias voltage is applied across the electrodes of this capacitor, irregularities on the surface of the backplate set up localized concentrated electric fields in the film. When a signal is superimposed on the dc bias during a transmission mode of operation, the film is stressed and oscillatory formations develop causing ultrasonic energy or an "acoustical" wavefront to be propagated from the film with its metallized surface, said combination also being referred to herein as a diaphragm. During the receive mode, variable ultrasonic pressure waves on the diaphragm deform the insulating film, thereby producing a variable voltage across said electrodes.
In the above-cited MUGGLI et al. patent, it is noted that transducer sensitivity to an echo of an ultrasonic pressure wave is improved by reducing transducer capacitance and that one way to reduce transducer capacitance is by sandblasting or roughening the transducer backplate surface that would otherwise contact the transducer diaphragm. For transducer capacitance repeatability in high volume transducer manufacturing operations, said MUGGLI et al. patent also describes a transducer backplate, diaphragm-contact surface having uniform striations.
A sandblasted or uniformly striated transducer backplate, diaphragm-contact surface will reduce transducer capacitance and improve transducer sensitivity as explained in said MUGGLI et al. patent. However, Applicant has observed that as the diaphragm-to-transducer contact surface is reduced by such surface texturing, for the purpose of increasing transducer sensitivity, etc., the time required for the vibrations of the transducer diaphragm to decay, after the termination of a transmit signal, increases. As noted above, for reliable distance measuring, increased vibration time or "ringing" necessarily increases the minimum object detection distance of a range finder system having a combination transmitting and receiving electrostatic transducer of the type described above.